SiCs (silicon carbides) are thermally and chemically very stable, superior in mechanical strengths, and resistant to radiation, and also have superior physical properties, such as high breakdown voltage and high thermal conductivity compared to Si crystals. They are therefore able to exhibit high output, high frequency, voltage resistance and environmental resistance that cannot be realized with existing single crystal substrate materials, such as Si crystals and GaAs crystals, and are being considered ever more promising as next-generation single crystal substrate materials for a wide range of applications including power device materials that allow high power control and energy saving to be achieved, device materials for high-speed large volume information communication, high-temperature device materials for vehicles, radiation-resistant device materials and the like.
The following production methods have been known in the prior art for fabrication of SiC single crystal substrates. First, a SiC ingot is obtained by crystal growth using a sublimation process or solution process. The SiC grown crystal is sliced from the SiC ingot by using a wire saw or the like, and then subjected to mirror polishing.
After cleaning the SiC single crystal substrate following mirror polishing, the SiC single crystal substrate is complete. The most important step in cleaning the SiC single crystal substrate is removal of the metal impurities on the crystal surface. When metal impurities remain on the crystal surface, they significantly affect the quality of the epitaxial film formed on the SiC single crystal substrate, potentially constituting a major hindrance to fabrication of an active component, such as a field-effect transistor on the substrate.
It is generally considered that properties near the theoretical value can be obtained if the metal atom content remaining on the surface of a SiC single crystal substrate is no greater than 1×1011 atoms/cm2, and immersing of SiC single crystal substrates in hydrochloric acid has been proposed as a method of removing the metal impurities on the surface of such SiC single crystal substrates (PTL 1).